Photochromic glasses, or phototropic glasses as they have been variously called, are glasses which darken under the influence of actinic radiation, normally ultraviolet radiation, and which return to the clear state when the actinic radiation is removed. U.S. Pat. No. 3,208,860, which is the basic patent in the field, describes a family of silicate glass compositions containing submicroscopic crystals of a silver halide dispersed throughout the glassy matrix, which crystals are reversibly darkenable under the action of ultraviolet radiation, imparting to the glass the characteristic of variable light transmission depending upon the amount of ultraviolet radiation present in radiation incident thereupon.
U.S. Pat. No. 3,197,296 describes alkali boroaluminosilicate glasses containing submicroscopic silver halide crystals which exhibit properties which are very desirable for ophthalmic applications. Hence the glasses described in that patent have refractive indices (n.sub.D) within the range of about 1.52-1.54, exhibit the desired degree of darkenability to be useful in ophthalmic thicknesses (about 2 millimeters), and demonstrate darkening and fading rates which are acceptable for ophthalmic lenses.
Most presently-available photochromic glasses utilized for ophthalmic and other applications exhibit a neutral gray color in the faded or unactivated state, and assume a darker but still neutral gray coloration upon activation with ultraviolet radiation. The color of these glasses is a function of the composition utilized to produce the photochromic glass, and of the thermal treatment utilized to convert the potentially-photochromic glass as formed into a photochromic article exhibiting the desired sensitivity to ultraviolet light.
Whereas prior art photochromic glasses for ophthalmic and other uses have been widely available in a gray coloration in the darkened and faded state, a demand for photochromic glasses of equivalent sensitivity and behavior, but exhibiting other colorations such as brown, has recently developed.
One method of modifying color in photochromic glasses comprises adding colorants to the base glass which are compatible with the photochromic constituents present therein. German Offenlegungsschrift No. 2,107,343, for example, described photochromic glasses containing additions of vanadium, chromium, manganese, and cobalt, which glasses exhibit a variety of colors in combination with photochromic properties. The use of colorants, however, necessitates changes in glass batch composition whenever a color modification is required, and the additives required to produce particular colors may add substantially to the cost of the batch.
It has also been proposed to subject photochromic glass of the silver halide type to "aftertreatments" which are designed to modify the color of the glass without substantial degradation of the photochromic properties thereof. Thus, U.S. Pats. Nos. 3,892,582 and 3,920,463 describe aftertreatments for already completely developed photochromic glasses wherein the glasses (containing microscopic particles of silver halide) are heated in a reducing atmosphere to impart yellow or brown coloration thereto. The need for supplemental treatments, however, can also add substantially to the cost of the photochromic product.
Thus there still exists a need for a simple, low-cost method for producing brown photochromic glass.